1. Field of the Invention
This invention relates to an electronic tachometer for producing a signal indicative of the speed at which a servo head traverses a series of magnetic tracks in data storage equipment, and more particularly to an improved differentiator for such tachometer that produces a highly accurate discontinuous speed signal in response to traversal of the magnetic tracks by the servo head.
2. Description of the Prior Art
Existent electronic tachometers for indicating the speed at which a servo head moves radially of a magnetic disk on which parallel concentric servo tracks are recorded are employed in many disk pack data storage systems. One known technique for producing such speed signal is to integrate the current signal applied to drive the servo head in the radial direction. Because the current is proportional to the acceleration of the servo head, integration of the current signal produces a signal representative of speed. Such signal, however, is subject to electronic integrator drift, external forces and force constant variation and therefore although continuous, is not sufficiently accurate.
Another known technique for producing a speed signal is to differentiate a signal representative of the position of the servo head relative to the tracks. This technique, although much more accurate than the integration technique, produces a discontinuous signal because the position signal, when the head moves at or near the center of one of the servo tracks, is not accurately related to head position.
The two techniques mentioned above have been combined to produce a tachometer that produces a continuous speed signal from an integrator which continuous signal is periodically reset or updated by a discontinuous speed signal produced by differentiating the position signal, U.S. Pat. No. 3,568,059 (324/177) to Sordello exemplifying such technique. Although the system disclosed in such patent provides improved performance over either of the above techniques when employed individually, present implementation utilizes an active differentiator that includes an operational amplifier. As is known, operational amplifiers have inherent characteristics such as amplifier slew rate, amplifier noise and band width limitations. Although these characteristics can be compensated for to avoid instability, they impair the accuracy of such system, particularly at low speeds wherein noise can account for a substantial proportion of the total output signal.
Further, the position signal includes alternate positive going and negative going portions, but portions of only one sense can be employed in producing a speed signal by differentiation. One known technique is to differentiate both the positive and negative going portions and then to rectify the resultant differentiated position signal so that all portions thereof vary in the same sense. Such rectification, however, increases the adverse effects of noise because the noise, rather than being averaged to zero by substantially equal positive and negative going portions is converted to DC by the rectifier. U.S. Pat. No. 3,883,894 (360/78) to Johnson exemplifies a system wherein the position signal is rectified.
Another technique, which eliminates the noise signal error resulting from rectification after differentiation is to provide two differentiators and to switch the differentiators so that the output of each differentiator is applied separately to the tachometer output during individual portions of the position signal. The accuracy of such system is dependent on the degree to which the components forming the differentiators are identical to one another. Identical components are impractical, particularly in commercially competitive systems. U.S. Pat. No. 3,820,712 (335/151.32) to Oswald exemplifies a system having two individual differentiators.